Electrical Discharge Coating Method and Green Compact Electrode Used Therein

ABSTRACT

An electrical discharge coating method comprising the steps of: generating pulsed discharge between a green compact electrode  3  and a treatment target surface  20  in a working fluid  10 ; thereby, depositing a component of the green compact electrode  3  onto the treatment target surface  20  so as to form a coating  21 , wherein the generating includes using the green compact electrode  3  which is formed through compression molding of metal powder having an oxide layer on each particle surface  20  thereof, as a main component thereof, the oxide layer being thicker than an oxide film normally obtained in air at normal temperature, whereby the metal component of the green compact electrode  3  is deposited onto the treatment target surface  20  so as to form the coating  21  containing the metal component as a main component thereof. It is possible to form a thick coating mainly containing a high hardness metal even on a low melting point metal such as an aluminum material, without depending on a carbide that reduces conductivity.

TECHNICAL FIELD

The present invention relates to an electrical discharge coating method for coating a surface of a treatment target through pulsed discharge on the treatment target using a green compact electrode of a metal powder or the like. In particular, the present invention relates to an electrical discharge coating method allowing coating of a low melting point metal, such as aluminum, with a hard thick coating, and also to a green compact electrode used therein.

BACKGROUND ART

Aluminum and its alloys (hereinbelow, simply referred to as an aluminum material) are light in weight and excellent in workability, but have drawbacks of poor wear resistance and corrosion resistance. Thus, anodization processes and vapor deposition processes such as PVD and CVD have been developed as surface treatment methods for coating a surface of an aluminum material with a harder coating. Also, to shield an aluminum material, which is poor in high-temperature strength, from high temperatures, coating processes such as plating and thermal spraying have been used industrially. However, anodization processes and plating have a disadvantage in that since they utilize electrochemical reactions in electrolytic solutions, it is difficult to partially treat an aluminum material. In addition, thermal spraying has a disadvantage in that since a large amount of heat is input into an aluminum material, the aluminum material is distorted by the heat. Moreover, vapor deposition processes such as PVD and CVD have a disadvantage of high equipment cost since a treatment furnace needs to be under vacuum.

In recent years, there has been developed a surface treatment method that resolves all these disadvantages; specifically, an electrical discharge coating method capable of easily performing partial treatment; requiring such a low heat input that no distortion may occur; and performing processes at normal pressure at low cost. This electrical discharge coating treatment forms a hard coating as follows (see Patent Document 1). Specifically, a green compact electrode is obtained through compression molding of a powder of metal, ceramic, or the like. An aluminum material is subjected to pulsed discharge using the green compact electrode in an organic working fluid. Thereby, a component of the green compact electrode is transferred and deposited onto a surface of the aluminum material.

The feature of this electrical discharge coating treatment is that, taking advantage of a dissociation phenomenon of an organic working fluid into the atomic level by a discharge energy, a component of the green compact electrode is reacted with carbon thus dissociated, and the resultant carbide is then transferred. Specifically, this treatment uses a green compact electrode obtained through compression molding of a uniformly-mixed powder of metal, such as titanium and niobium, which are easy to carbonize. This green compact electrode is placed to face an aluminum material in an organic working fluid such as kerosene and is subjected to pulsed discharge, so that the electrode material and its carbide are transferred onto the surface of the aluminum material. Accordingly, a coating containing a high hardness carbide as its main component, and thus having a high wear resistance, can be formed.

The electrical discharge coating treatment as above improves the wear resistance due to the high hardness carbide, but has a problem of not being able of yield such a thick coating that the aluminum material can be shielded from high temperatures. This is attributable to the fact that as a ratio of carbide on the coating surface increases along with the progress of the coating deposition, the conductivity of the coating surface decreases, and thus the pulsed discharge fails to occur properly. Even if pulsed discharge is caused by applying a higher voltage, the carbide on the coating surface, which has a high melting point and poor in reactivity, hinders transfer of eluted electrode material, making it difficult to achieve thick coating formation.

To address this problem, Patent Document 2 discloses a method for achieving thick coating formation as follows, although the treatment target is not an aluminum material. Specifically, a green compact electrode is obtained by mixing a powder of a metal that carbonizes easily, such as titanium or niobium, with a powder of a metal, such as cobalt, that carbonizes less easily than the former metal. Using the mixed metal powders, an electrical discharge coating treatment is performed on a steel material surface to thereby increase the amount of a material which remains in a coating in the metal form.

However, when a low melting point metal such as an aluminum material is used in the above electrical discharge coating treatment using the green compact electrode in which the less-easily-carbonizing metal is mixed, the aluminum material is eluted by the heat of electrical discharge more than the electrode material is eluted. Hence, the transferring itself of the electrode material and its carbide onto the aluminum material surface is difficult.

If the application voltage is lowered and the pulse width is increased so as to suppress elution of the aluminum material, the reaction time with carbon atoms dissociated from the working fluid becomes longer, which accordingly increases a carbonizing ratio even when the metal is difficult to carbonize. Thus, thick coating formation cannot be expected for the aforementioned reason. In addition, as the pulse width increases, the amount of the green compact electrode eluted by pulsed discharge at a time becomes larger. This leads to deposition of the carbide in the form of a large mass, and thus a coating including many defects is formed.

[Patent Document 1] JP 2002-242621 A

[Patent Document 2] JP 7-197275 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of these circumstances and has an object to provide: an electrical discharge coating method allowing a high-quality, thick coating mainly containing a high hardness metal to be formed even on a low melting point metal such as an aluminum material, without depending on a carbide that lowers conductivity; and a green compact electrode used in the method.

Means for Solving the Problems

To solve the above problems, the inventor earnestly researched, and consequently achieved the present invention through the following findings. Specifically, even in a case of powder of a high melting point metal which allows formation of a highly heat-resistant and wear-resistant coating, the metal particles can be eluted at a lower temperature than the melting point of the constituting metal, as long as a significant oxide layer is formed on a surface thereof. Hence, a coating can be formed without a large amount of heat input into a treatment target surface.

Specifically, the present invention is an electrical discharge coating method for forming a coating by generating pulsed discharge between a green compact electrode and a treatment target surface in a working fluid and by depositing a component of the green compact electrode onto the treatment target surface, the electrical discharge coating method wherein

the used green compact electrode is formed through compression molding of a metal powder as a main component thereof, the metal powder including an oxide layer on a surface thereof, the oxide layer being thicker than an oxide film normally obtained in air at normal temperature, and

a metal component of the green compact electrode is deposited onto the treatment target surface to thereby form the coating containing the metal component as a main component thereof.

In addition, the present invention is a green compact electrode used for the electrical discharge coating treatment, and is formed through compression molding of a metal powder as a main component thereof, the metal powder including an oxide layer on a surface thereof, the oxide layer being thicker than an oxide film normally obtained in air at normal temperature.

EFFECTS OF THE INVENTION

According to the present invention, a green compact electrode is obtained through compression molding of a metal powder as a main component thereof, the metal powder including, on a surface thereof, an oxide layer having a significant thickness. The use of the green compact electrode allows metal particles to be eluted therefrom at a temperature far lower than the melting point of the metal itself and deposited onto a treatment target surface for coating formation. Accordingly, elution and deposition of the metal component can be performed by pulsed discharge in such a short period of time that no carbide with the metal component may be produced. A coating can therefore be formed without a large amount of heat input into the treatment target surface. Consequently, a highly heat-resistant and wear-resistant coating mainly containing a hard and high melting point metal can be formed even on a low melting point metal such as an aluminum material.

Moreover, a coating can be formed mainly with a metal component, instead of allowing the coating form to depend on a carbide. Thus, no decrease in conductivity of the coating surface is induced, whereby a thick coating is obtained. Additionally, a uniform coating can be formed without variation of its component in the thickness direction. Also, because the coating form is not dependent on carbide, a working fluid other than an organic working fluid can be used, as long as it is an insulating working fluid which does not react with the metal component. Accordingly, it is also possible to form a coating containing no carbide, irrespective of the electrical discharge conditions.

In the present invention, the metal powder is preferably a powder of one metal selected from the group consisting of molybdenum (Mo), tungsten (W), chromium (Cr), molybdenum alloys, tungsten alloys, and chromium alloys. Meanwhile, the metal powder may be a mixed powder of two or more metals selected from the group consisting of molybdenum, tungsten, chromium, molybdenum alloys, tungsten alloys, and chromium alloys. Any of these metals from Group 6 of the periodic table and their alloys may be used alone to form a highly heat-resistant and wear-resistant coating.

Furthermore, the green compact electrode of the present invention is preferably formed through compression molding of the metal powder with a metal soap added thereto. The addition of a metal soap of course improves the moldability of the green compact electrode, but also promotes elution of the metal component at the time of the electrical discharge coating treatment. This is useful when a coating is formed by performing pulsed discharge for a short period of time while suppressing heat input into the treatment target surface.

Furthermore, the green compact electrode of the present invention is preferably formed through compression molding of the metal powder with a copper powder or a silver powder added thereto. The addition of a copper powder or a silver powder makes uniform the conductivity of the porous green compact electrode and therefore allows uniform pulsed discharge that occurs between the electrode and the treatment target surface. This is useful in preventing defects in the coating due to localized electrical discharge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an outline of an electrical discharge coating apparatus for implementing an electrical discharge coating method according to the present invention.

FIG. 2 is a graph showing the relationship between an oxidized amount of metal powder and a coating thickness.

FIG. 3 is a SEM cross-sectional image showing a coating (A) of a first example of the present invention.

FIG. 4 is a SEM cross-sectional image showing a coating of a comparative example.

FIG. 5 is a SEM cross-sectional image showing a coating by a conventional method.

FIG. 6 is a graph showing the relationship between an added amount of a metal soap (zinc stearate) and a coating thickness.

FIG. 7 is a SEM cross-sectional image showing a coating (A2) of a second example of the present invention.

FIG. 8 is a graph showing a result of X-ray diffraction of the coating (A2) of the second example of the present invention.

FIG. 9( a) is a SEM image of a coating near a defect. FIGS. 9( b) and 9(c) are pictures showing a carbon distribution and an oxygen distribution, respectively, obtained by EDS analyses in the same area as that in FIG. 9( a).

FIG. 10 is a SEM cross-sectional image showing a coating (A3) of a third example of the present invention.

FIG. 11( a) is a reduced-scale SEM cross-sectional image in FIG. 10. FIGS. 11( b) to 11(f) are pictures showing distributions of carbon, oxygen, aluminum, copper, and molybdenum, respectively, obtained by EDS analyses in the same area as that in FIG. 11( a).

FIG. 12 is a graph showing the Vickers hardness of the coating (A3) of the third example of the present invention at each measuring point.

FIG. 13 is a graph showing the relationship between an oxidized amount of a mixed powder of a molybdenum powder with a chromium powder and a coating thickness on a magnesium material surface.

FIG. 14 is a SEM cross-sectional image showing a coating (A4) of a fourth example of the present invention.

FIG. 15 is a SEM cross-sectional image showing a coating of a comparative example.

FIG. 16 is a graph showing the relationship between an added amount of a metal soap (zinc stearate) and a coating thickness.

FIG. 17 is a SEM cross-sectional image showing a coating (A5) of a fifth example of the present invention.

FIG. 18 is a graph showing the relationship between an oxidized amount of a mixed powder of a molybdenum powder with a chromium powder and a coating thickness on a titanium material surface.

FIG. 19 is a SEM cross-sectional image showing a coating (A6) of a sixth example of the present invention.

FIG. 20 is a SEM cross-sectional image showing a coating of a comparative example.

FIG. 21 is a graph showing the relationship between an added amount of a metal soap (zinc stearate) and a coating thickness.

FIG. 22 is a SEM cross-sectional image showing a coating (A7) of a seventh example of the present invention.

EXPLANATION OF REFERENCE NUMERALS

-   1 treatment tank -   2 treatment target -   3 green compact electrode -   4 power supply unit -   5 support -   10 working fluid -   20 treatment target surface -   21 coating -   30 electrical discharge surface -   31 shaft part

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an outline of an electrical discharge coating apparatus for implementing a method of the present invention. In FIG. 1, the electrical discharge coating apparatus includes: a treatment tank 1 to pool a working fluid 10; a green compact electrode 3 placed to face a treatment target 2 accommodated while immersed in the working fluid 10; a power supply unit 4 and power lines to apply voltage pulses between the treatment target 2 and the green compact electrode 3; and the like. The treatment target 2 is connected to the cathode of the power supply unit 4 and the green compact electrode 3 is connected to the anode thereof.

The green compact electrode 3 is bonded and fixed to the tip of a shaft part 31 made of a good conductor, such for example as copper, with a conductive adhesive therebetween. At the base (upper end portion) of the shaft part 31, the green compact electrode 3 is detachably held by a chuck 51 of a support 5. The support 5 includes mechanisms for movement and positioning at least in a direction of one vertical axis, and more preferably, mechanisms for movement and positioning along three orthogonal axes including the one vertical axis, as well as a control unit for the mechanisms, though these are not illustrated here. The support 5 is therefore designed to be capable of freely positioning an electrical discharge surface 30 at the tip of the green compact electrode 3 with respect to any part of a treatment target surface 20 of the treatment target 2.

In manufacturing of the green compact electrode 3, a metal powder as a raw material is heated under an atmosphere in advance to form an oxide layer thereon. The oxide layer is formed thicker than an oxide film that is normally obtained at a normal temperature. Then, as a main component, the metal powder subjected to such a surface oxidation treatment is compression-molded in a mold. As the raw material metal powder, a powder of a hard and high melting point metal from Group 6 of the periodic table is preferable, such as molybdenum (Mo), tungsten (W), or chromium (Cr), from each of which a highly heat-resistant and wear-resistant coating can be formed.

These metals may be used alone, or in combination of two or more thereof uniformly mixed together. In addition, a powder of an alloy having any of the above metals as its main component, i.e., a molybdenum alloy, a tungsten alloy, a chromium alloy, or the like may be used alone or in combination. Since the oxidation treatment described above is performed at a temperature significantly lower than the melting points of these metals, the metal powders may be mixed either before or after the oxidation treatment.

The metal forming the treatment target 2 is not particularly limited. The present invention can be carried out on various kinds of metal materials. Nonetheless, the present invention is especially suitable for formation of a coating on a surface of a material made of aluminum, an aluminum alloy, magnesium, a magnesium alloy, titanium, or a titanium alloy, formation of a hard, thick coating on which has been difficult conventionally. In addition, as the working fluid 10, an organic working fluid such as a mineral oil can be used as in a case of a conventional treatment. However, a working fluid other than an organic working fluid can be used, as long as it is an insulating working fluid which does not react with a metal component.

In an electrical discharge coating treatment, the power supply unit 4 applies voltage pulses between the green compact electrode 3 and the treatment target 2 immersed in the working fluid 10, whereby pulsed discharge occurs between the electrical discharge surface 30 of the green compact electrode 3 and the treatment target surface 20. By the discharge energy thus generated, metal particles constituting the green compact electrode 3 are eluted and deposited onto the treatment target surface 20, whereby a coating 21 is formed.

As a condition for the electrical discharge, a short pulse width is selected so that the eluted metal component would not react with carbon in the working fluid. By using the green compact electrode 3 including, as its main component, a metal powder provided with an oxide layer on a surface thereof, an oxide is interposed between at least parts of contact surfaces of the metal particles constituting the green compact electrode 3. This weakens the bonds between the metal particles and also increases the electric resistance at the contact surfaces of the metal particles, thereby increasing localized heat generation. Accordingly, pulsed discharge for a short period of time enables elution and deposition of the metal component. Furthermore, in a case of molybdenum, tungsten, or an alloy thereof, the melting point of surface parts of the metal particles is lowered due to the oxide layers on the surfaces. Thus, the metal particles can be eluted at a lower temperature than the melting point of the metal itself. This also contributes to transfer of the metal component by the short-time pulsed discharge.

When the metal component is eluted and deposited onto the treatment target surface 20 by the short-time pulsed discharge as described above, the coating 21 can be formed without a large amount of heat input into the treatment target surface 20. Accordingly, a highly heat-resistant and wear-resistant coating 21 containing a hard and high melting point metal as its main component can be formed even on a treatment target surface 20 made of a low melting point metal, such as an aluminum material, while suppressing melting of the treatment target surface 20. Moreover, since the coating 21 mainly containing the metal component is formed through the short-time pulsed discharge producing no carbide of the metal component, no decrease in conductivity of the coating surface is induced. Thus, a thick and homogenous coating 21 can be obtained by a brief treatment.

The ratio of the oxide layer formed on the surface of the metal powder constituting the green compact electrode 3 is preferably 4 to 14% by weight of the metal powder, although the ratio varies somewhat depending on the average particle diameter of the metal powder. No contribution to thick coating formation is found if the oxide layer is below 4% by weight. In general, the metal powder serving as a raw material includes an oxide film in a stable passive state on its surface due to oxidation at normal temperature, but such an oxide film is so thin that it does not contribute to thick coating formation.

The oxide layer being not less than 4% by weight of the metal powder shows a remarkable effect on thick coating formation, but an excessively high ratio of the oxide layer causes the coating to include many defects, making it impossible to obtain a high quality coating. For this reason, in formation of a high-quality thick coating, the ratio of the oxide layer is preferably controlled to fall within a range from 5 to 11% by weight of the metal powder.

EXAMPLES

To test the above-described influence on coating thickness of the oxide layer on the metal powder surface, the following experiments were carried out. Specifically, an electrical discharge coating treatment was performed on surfaces of an aluminum material, a magnesium material, and a titanium material while changing only the amount of oxidation of metal powder but without changing the formation conditions of a green compact electrode and the electrical discharge conditions thereof. Then, changes in coating thickness after the treatment were examined.

The experiment on the surface of the aluminum material was carried out as follows. A molybdenum powder having an average particle diameter of 2 μm was used as the metal powder. Columnar green compacts each having a diameter of 13.8 mm were prepared at a molding pressure of 400 MPa while changing the oxidized amount of the metal powder in a range from 3 to 14% by weight through an oxidation treatment performed under atmosphere at 250° C. With the green compacts as negative electrodes, electrical discharge coating treatments were each performed on a surface of an aluminum alloy (A2017) in a working fluid (EDF-K of Nippon Oil Corporation) for 2 minutes, under electrical discharge conditions of a peak current value of 20 A, a pulse width of 50 μsec, and a duty ratio of 18%. Then, the coating thickness of each obtained coating was measured. Incidentally, as a comparative example, an experiment was performed under the same conditions with a molybdenum powder which included only an oxide film generally obtained in air at normal temperature and which had an oxygen content of 0.4% by weight. Then, the coating thickness of the obtained coating was measured.

FIG. 2 is a graph showing the result of the experiment. According to this result, the oxidized amount of the molybdenum powder shows almost no influence on thick coating formation when below 4% by weight. On the other hand, once the oxidized amount exceeds 4% by weight, the coating thickness increases rapidly. The coating thickness peaks at an oxidized amount of 11% by weight, reaching 1100 μm or more. The coating thickness shows a tendency to decrease after the peak; nonetheless, the obtained coating is still thicker than in a usual case. However, it was found that when the oxidized amount was 14% by weight or more, the coating happened to include many defects.

Similar experiments were carried out using powders of tungsten and chromium as the metal powders. In these experiments too, thick coatings with small amounts of defects were obtained by adjusting the oxidized amounts within a range from 5 to 11% by weight. From these results, the electrical discharge coating method of the present invention makes it possible to perform treatment without a generally-used high-purity expensive metal powder which is low in oxidized amount. The electrical discharge coating method is also advantageous in cost. Additionally, the oxidation treatment can be performed in a metal-powder preparing step.

FIG. 3 is a cross-sectional image showing a coating (referred to as a coating A of a first example of the present invention) that is subjected to the electrical discharge coating treatment for 2 minutes using the green compact electrode of the molybdenum powder having an oxidized amount of 11% by weight in the above experiment. It can be observed that the coating thickness is at least 1100 μm, and reaches 1200 μm or above at a thicker part. Also, the coating is formed uniformly in its thickness direction.

FIG. 4 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating B) that is subjected to the electrical discharge coating treatment for 2 minutes using the green compact electrode of the molybdenum powder having an oxidized amount of 14% by weight in the above experiment. It can be observed that the coating thickness is approximately 500 μm. The coating can be said to be much thicker than one obtained by a conventional electrical discharge coating treatment. However, there was observed an increase in internal defects.

FIG. 5 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating C) that is subjected to the electrical discharge coating treatment using the green compact electrode of the molybdenum powder used in the above experiment as a comparative example, the molybdenum powder having an oxygen content of 0.4% by weight. The coating thickness stays around 80 μm. In addition, an interface of the coating and the aluminum material is somewhat rough as compared to the coatings A and B. This might be attributable to the fact that molybdenum may reach the aluminum material's surface at a higher temperature than that in the case in which the oxide layer is present on the surface of the molybdenum powder, and such heat may cause melting or heat deformation of the aluminum material's surface.

Next, an experiment was carried out as follows to examine effects on coating thickness in a case in which a metal soap is added to the metal powder as a raw material of the green compact electrode and compression-molded.

As the metal soap, zinc stearate was used. As the metal powder, used was the molybdenum powder having an oxidized amount of 11% by weight from which the thickest coating (coating A) was obtained in the above experiment. The amount of zinc stearate added to the powder was changed in a range from 0 to 6% by weight. Then, each of the powders thus blended was mixed for 60 minutes using a V-type mixer so that each mixed powder would be uniform. These mixed powders were compression-molded to manufacture green compact electrodes. An electrical discharge coating treatment was performed for each case for 2 minutes under the same conditions as the above experiment. Then, the coating thickness of each obtained coating was measured. The result thereof is shown in FIG. 6.

As shown in the graph in FIG. 6, the coating thickness was greatly improved when zinc stearate was added at 1% by weight, and reached the largest (2100 μm) when zinc stearate was added at 2% by weight. Addition at more than 2% by weight showed an improvement in coating thickness as compared to a case without any addition. However, it was observed that addition at 5% by weight or more increased the consumption of the green compact electrode. This might be attributable to the fact that although improving moldability, the addition of zinc stearate may excessively weaken the bonds between the metal particles, and the metal particles may be released from the electrode without being melted sufficiently and failed to be incorporated into the coating component.

Similar experiments were carried out using tungsten and chromium. It was observed that the coating thickness were highest when zinc stearate was added at 2% by weight, and the consumption of the green compact electrode became more intense when zinc stearate was added at 6% by weight or more. From these facts, the amount of the metal soap added is preferably 1 to 4% by weight, although addition by 1% by weight or less may still contribute to thick coating formation.

FIG. 7 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating A2 of a second example of the present invention) obtained by adding the zinc stearate at 2% by weight which contributed the formation of the thickest coating in the above experiment. Although the coating includes small defects, thick coating formation approximately twice as thick as in a case without any addition is achieved. FIG. 8 is a graph showing a result of X-ray diffraction (XRD) of the coating A2. According to FIG. 8, it was observed that the main component of the coating A2 of the second example is molybdenum (Mo), and carbide thereof (Mo₂C) was very scarce.

Note that as the metal soap, various metal soaps can be used, such as a stearic acid soap of a metal salt other than zinc stearate, as well as 12-hydroxystearic acid soap, montanic acid soap, behenic acid soap, and lauric acid soap. Any of these has a low melting point and therefore disappears through vaporization or the like when metal particles are eluted by electrical discharge. Thus, influence of the metal soap on coating form is small.

As described above, thick coating formation of 1000 μm or more was achieved by the electrical discharge coating treatment using a green compact electrode of a metal powder having a significant oxide layer on a surface thereof. Furthermore, thick coating formation of 2000 μm or more was achieved by addition of a metal soap. On the other hand, in a case of a sample in which the oxidized amount of the metal powder is large or the added amount of the soap metal is large, defects are sometimes left inside the coating, as mentioned above.

To find out the reason, elements near defects in a coating (hereinbelow, referred to as a coating A2′) obtained by adding zinc stearate at 4% by weight were identified through an energy-dispersive X-ray elemental analysis (EDS analysis). FIG. 9 shows the result of the EDS analysis.

FIG. 9( a) is a SEM image near a defect. FIGS. 9( b) and 9(c) show a carbon distribution and an oxygen distribution in the same area, respectively. As shown in FIG. 9( b), a large amount of carbon was detected near the defect. When the XRD result regarding the coating A2 shown in FIG. 8 is considered although the conditions are somewhat different, it is difficult to think of the carbon as a carbide of molybdenum. Rather, the carbon is assumed to be carbon which originates from the organic working fluid and is incorporated into the coating after being released by abnormal pulsed discharge. The abnormal pulsed discharge occurs due to a non-uniform conductivity of the coating surface.

Thus, a metal powder excellent in conductivity, e.g., a copper powder, should be added to the metal powder serving as the main component of the green compact electrode. In this way, the conductivity of the coating surface is made uniform and pulsed discharge is stabilized. This prevents defects in the coating and thus improves coating form.

In this respect, an electrical discharge coating treatment was performed under the same conditions as the above experiment using a green compact electrode manufactured from a mixed powder obtained by further adding a copper powder to the mixed powder (obtained by adding zinc stearate, at 2% by weight, to the molybdenum powder having an oxidized amount of 11% by weight) used in the formation of the coating A2 of the second example. Then, defects in the coating and changes in coating thickness with respect to copper added amounts (0 to 20% by weight) were examined. As a result, favorable coatings with few defects were obtained when the copper powder was added at 2 to 4% by weight. However, when the copper powder was added at 4% by weight or more, defects were prevented, but a large decrease in coating thickness was observed. Meanwhile, similar experiments were performed using tungsten and chromium. In these experiments too, it was observed that favorable coatings were obtained when the copper powder was added at 2 to 4% by weight.

FIG. 10 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating A3 of a third example of the present invention) subjected to an electrical discharge coating treatment performed under the same conditions as each above experiment using a green compact electrode of a mixed powder obtained by: adding zinc stearate, at 2% by weight, to a molybdenum powder having an oxidized amount of 11% by weight; further adding a copper powder at 4% by weight; and then uniformly mixing the powders thus blended. From this image, it can be observed that the coating A3 of the third example has homogeneous and dense coating form as compared to the coating A2 of the second example obtained by using the green compact electrode with no copper added thereto.

A result of an EDS analysis of the same area as that in FIG. 10 for the coating A3 of the third example is shown in FIGS. 11( a) to 11(f). FIG. 11 (a) is a reduced-scale SEM cross-sectional image of the same area as that in FIG. 10, and FIGS. 11( b) to 11(f) show distributions of carbon (b), oxygen (c), aluminum (d), copper (e), and molybdenum (f), respectively, in the same area. From these, it can be observed that in the coating A3, there is almost no variation in component including carbon in the cross-sectional direction.

Next, a hardness test was carried out using a Vickers hardness meter to check the hardness of the coating A3 of the third example. In the hardness test, the coating A3 was ground until its thickness was 200 μm, 800 μm, and 1700 μm from the interface with an aluminum material (A2017), and then the Vickers hardnesses against a load of 50 gf were measured. The results of the tests are shown in FIG. 12. Note that a measurement point at the left end in FIG. 12 indicates the hardness of the aluminum material itself.

As shown in FIG. 12, the hardness of the coating A3 of the third example reached 450 to 470 HV, showing that a very hard coating was achieved. The coating by the electrical discharge coating method of the present invention is uniform in component distribution in its thickness direction as mentioned earlier, and furthermore, like the component distribution, the hardness distribution also is found to hardly change in the cross-sectional direction.

Next, an experiment on the surface of the magnesium material was carried out as follows. As the metal powder, a molybdenum powder having an average particle diameter of 2 μm and a chromium powder having an average particle diameter of 10 μm were used. An oxidation treatment was performed on both of the powders under atmosphere at 250° C. while changing the oxidized amount in a range from 3 to 14% by weight. Thereafter, the powders were mixed together in such a manner that the chromium powder would be 17% by weight in the ratio of the molybdenum powder and the chromium powder. Using the mixed metal powder, a green compact was prepared as in the case of the experiment on the surface of the aluminum material. Electrical discharge coating treatments were each performed on the surface of a magnesium alloy (AZ91) for 2 minutes under the same electrical discharge conditions as the case of the surface of the aluminum material. Then, the coating thickness of each obtained coating was measured.

FIG. 13 is a graph showing the result of the experiment. According to this result, the oxidized amount of the mixed metal powder showed almost no influence on thick coating formation when below 4% by weight. On the other hand, once the oxidized amount exceeded 4% by weight, the coating thickness increases rapidly. The coating thickness peaks at an oxidized amount of 11% by weight, reaching 1300 μm or above. The coating thickness lacks uniformity after the peak; nonetheless, the obtained coating is still thicker than a usual case. However, it was found that when the oxidized amount was 14% by weight or more, the coating happened to include many defects.

Similar experiments were carried out using powders of tungsten and chromium as the green compact raw material powders. In these experiments too, thick coatings with small amounts of defects were obtained by adjusting the oxidized amounts within a range from 5 to 11% by weight. From these results, the electrical discharge coating method of the present invention makes it possible to perform treatment without a generally-used high-purity expensive metal powder which is low in oxidized amount. The electrical discharge coating method is also advantageous in cost. Additionally, the oxidation treatment can be performed in a metal-powder preparing step.

FIG. 14 is a cross-sectional image showing a coating (referred to as a coating A4 of a fourth example of the present invention) that is subjected to the electrical discharge coating treatment for 2 minutes using the green compact electrode of the mixed metal powder having an oxidized amount of 11% by weight in the above experiment. It can be observed that the coating thickness is at least 1200 μm, and reaches 1300 μm or above at a thicker part. Also, the coating is formed uniformly in its thickness direction.

FIG. 15 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating B4) that is subjected to the electrical discharge coating treatment for 2 minutes using the green compact electrode of the mixed metal powder having an oxidized amount of 14% by weight in the above experiment. It can be observed that the coating thickness is approximately 1000 to 1500 μm. The coating can be said to be much thicker than one obtained by a conventional electrical discharge coating treatment. However, there is observed an increase in internal defects.

Next, an experiment was carried out as follows to examine effects on coating thickness in a case in which a metal soap is added to the mixed metal powder as a raw material of the green compact electrode and compression-molded.

As the metal soap, zinc stearate was used. As the metal powder, used was the mixed metal powder having an oxidized amount of 11% by weight from which the thickest coating (coating A4) was obtained in the above experiment. The amount of zinc stearate added to the powder was changed in a range from 0 to 6% by weight. Then, each of the powders thus blended was mixed for 60 minutes using a V-type mixer so that each mixed powder would be uniform. These mixed powders were compression-molded to manufacture green compact electrodes. An electrical discharge coating treatment was performed for each case for 2 minutes under the same conditions as the above experiment. Then, the coating thickness of each obtained coating was measured. The results thereof are shown in FIG. 16.

As shown in the graph in FIG. 16, the coating thickness was greatly improved when zinc stearate was added by 1% by weight, and reached the largest (2600 μm) when zinc stearate was added by 2% by weight. Addition by more than 2% by weight showed an improvement in coating thickness as compared to a case without any addition. However, it was observed that addition by 5% by weight or more increased the consumption of the green compact electrode. This might be attributable to the fact that although improving moldability, the addition of zinc stearate may excessively weaken the bonds between the metal particles, and the metal particles may be released from the electrode without being melted sufficiently and failed to be incorporated into the coating component.

Similar experiments were carried out using tungsten and chromium. It was observed that the coating thickness was greatest when zinc stearate was added at 2% by weight, and the consumption of the green compact electrode became more intense when zinc stearate was added at 6% by weight or more. From these facts, the amount of the metal soap added is preferably 1 to 4% by weight, although addition by 1% by weight or less may still contribute to thick coating formation.

Note that as the metal soap, various metal soaps can be used, such as a stearic acid soap of a metal salt other than zinc stearate, as well as 12-hydroxystearic acid soap, montanic acid soap, behenic acid soap, and lauric acid soap. Any of these has a low melting point and therefore disappears through vaporization or the like when metal particles are eluted by electrical discharge. Thus, influence of the metal soap on coating form is small.

Next, an electrical discharge coating treatment was performed under the same conditions as the above experiment using a green compact electrode manufactured from a mixed powder obtained by further adding a silver powder to a mixed powder (obtained by adding zinc stearate, by 2% by weight, to the mixed metal powder having an oxidized amount of 11% by weight) from which the thickest coating was obtained in the above experiment. Then, defects in the coating and changes in coating thickness with respect to silver added amounts (0 to 20% by weight) were examined. As a result, favorable coatings with few defects were obtained when the silver powder was added at 2 to 4% by weight. However, when the silver powder was added at 4% by weight or more, defects were prevented but a large decrease in coating thickness was observed. Meanwhile, similar experiments were performed using tungsten and chromium. In these experiments too, it was observed that favorable coatings were obtained when the silver powder was added at 2 to 4% by weight.

FIG. 17 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating A5 of a fifth example of the present invention) subjected to an electrical discharge coating treatment performed under the same conditions as each above experiment using a green compact electrode of a mixed powder obtained by: adding zinc stearate, at 2% by weight, to the mixed metal powder having an oxidized amount of 11% by weight; further adding a silver powder at 4% by weight; and then uniformly mixing the powders thus blended. From this image, it can be observed that the coating had a homogeneous and dense form.

Next, an experiment on the surface of the titanium material was carried out as follows. As the metal powder, a molybdenum powder having an average particle diameter of 2 μm and a chromium powder having an average particle diameter of 10 μm were used. An oxidation treatment was performed on both of the powders under an atmosphere at 250° C. while changing the oxidized amount in a range from 3 to 14% by weight. Thereafter, the powders were mixed together in such a manner that the chromium powder would be 17% by weight in the ratio of the molybdenum powder and the chromium powder. Using the mixed metal powder, a green compact was prepared as in the case of the experiment on the surface of the aluminum material. Electrical discharge coating treatments were performed on a surface of a pure titanium material for 2 minutes under the same electrical discharge conditions as the case of the surface of the aluminum material. Then, the coating thickness of each obtained coating was measured.

FIG. 18 is a graph showing the result of the experiment. According to this result, the oxidized amount of the mixed metal powder shows almost no influence on thick coating formation when below 4% by weight. On the other hand, once the oxidized amount exceeds 4% by weight, the coating thickness increased rapidly. The coating thickness peaks at an oxidized amount of 11% by weight, reaching 1300 μm or above. Even after the peak, the obtained coating is still thicker than a usual case. However, it was found that when the oxidized amount was 14% by weight or more, the coating happened to include many defects.

Similar experiments were carried out using powders of tungsten and chromium as the green compact raw material powders. In these experiments too, thick coatings with small amounts of defects were obtained by adjusting the oxidized amounts within a range from 5 to 11% by weight. From these results, the electrical discharge coating method of the present invention makes it possible to perform treatment without a generally-used high-purity expensive metal powder which is low in oxidized amount. The electrical discharge coating method is also advantageous in cost. Additionally, the oxidation treatment can be performed in a metal-powder preparing step.

FIG. 19 is a cross-sectional image showing a coating (referred to as a coating A6 of a sixth example of the present invention) that is subjected to the electrical discharge coating treatment for 2 minutes using the green compact electrode of the mixed metal powder having an oxidized amount of 11% by weight in the above experiment. It can be observed that the coating thickness is at least 1200 μm, and reaches 1300 μm or above at a thicker part. Also, the coating is formed uniformly in its thickness direction.

FIG. 20 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating B6) that is subjected to the electrical discharge coating treatment for 2 minutes using the green compact electrode of the mixed powder having an oxidized amount of 14% by weight in the above experiment. It can be observed that the coating thickness is approximately 1000 to 1500 μm. The coating can be said to be much thicker than one obtained by a conventional electrical discharge coating treatment. However, there was observed an increase in internal defects.

Next, an experiment was carried out as follows to examine effects on coating thickness in a case in which a metal soap is added to the mixed metal powder as a raw material of the green compact electrode and compression-molded.

As the metal soap, zinc stearate was used. As the metal powder, the mixed metal powder having an oxidized amount of 11% by weight from which the thickest coating (coating A6) was obtained in the above experiment was used. The amount of zinc stearate added to the powder was changed in a range from 0 to 6% by weight. Then, each of the powders thus blended was mixed for 60 minutes using a V-type mixer so that each mixed powder would be uniform. These mixed powders were compression-molded to manufacture green compact electrodes. An electrical discharge coating treatment was performed for each case for 2 minutes under the same conditions as the above experiment. Then, the coating thickness of each obtained coating was measured. The result thereof is shown in FIG. 21.

As shown in the graph in FIG. 21, the coating thickness was greatly improved when zinc stearate was added at 1% by weight, and reached the greatest (2350 μm) when zinc stearate was added by 2% by weight. Addition at more than 2% by weight showed an improvement in coating thickness as compared to a case without any addition. However, it was observed that addition at 5% by weight or more increased the consumption of the green compact electrode. This might be attributable to the fact that although improving moldability, the addition of zinc stearate may excessively weaken the bonds between the metal particles, and the metal particles may be released from the electrode without being melted sufficiently and failed to be incorporated into the coating component.

Similar experiments were carried out using tungsten and chromium. It was observed that the coating thickness reached the largest when zinc stearate was added at 2% by weight, and the consumption of the green compact electrode became more intense when zinc stearate was added at 6% by weight or more. From these facts, the amount of the metal soap added is preferably 1 to 4% by weight, yet addition at 1% by weight or less may still contribute to thick coating formation.

Note that as the metal soap, various metal soaps can be used, such as a stearic acid soap of a metal salt other than zinc stearate, as well as 12-hydroxystearic acid soap, montanic acid soap, behenic acid soap, and lauric acid soap. Any of these has a low melting point and therefore disappears through vaporization or the like when metal particles are eluted by electrical discharge. Thus, effects of the metal soap on coating form are small.

Next, an electrical discharge coating treatment was performed under the same conditions as the above experiment using a green compact electrode manufactured from a mixed powder obtained by further adding a silver powder to a mixed powder (obtained by adding zinc stearate, by 2% by weight, to the mixed metal powder having an oxidized amount of 11% by weight) from which the thickest coating was obtained in the above experiment. Then, defects in the coating and changes in coating thickness with respect to silver added amounts (0 to 20% by weight) were examined. As a result, favorable coatings with few defects were obtained when the silver powder was added at 2 to 4% by weight. However, when the silver powder was added at 4% by weight or more, defects were prevented, but a large decrease in coating thickness was observed. Meanwhile, similar experiments were performed using tungsten and chromium. In these experiments too, it was observed that favorable coatings were obtained when the silver powder was added by 2 to 4% by weight.

FIG. 22 is a cross-sectional image showing a coating (hereinbelow, referred to as a coating A7 of a seventh example of the present invention) subjected to an electrical discharge coating treatment performed under the same conditions as each above experiment using a green compact electrode of a mixed powder obtained by: adding zinc stearate, at 2% by weight, to the mixed metal powder having an oxidized amount of 11% by weight; further adding a silver powder at 4% by weight; and then uniformly mixing the powders thus blended. From this image, it can be observed that the coating had homogeneous and dense form.

It should be noted that although the above-described examples each have been shown for the case in which the treatment target 2 is an aluminum alloy, a magnesium alloy, or pure titanium, the present invention is not limited thereto. The present invention can be carried out on a surface of aluminum, magnesium, or a titanium alloy as a matter of course, and additionally on surfaces of various metals other than these.

As has been described above, the electrical discharge coating method of the present invention makes it possible to form a thick coating of a hard and high melting point metal on a surface of a low melting point metal such as an aluminum material in a short period of time. In each of the second and third examples, a thick coating having a thickness of 1600 to 2000 μm is formed through an electrical discharge coating treatment for only 2 minutes, achieving remarkable thick coating formation that is 10 to 100 times thicker than conventional cases. In addition, the coating formation rate has reached 800 to 1000 μm/min, allowing thick coating formation, and at the same time, a significant reduction in treatment time, i.e., high speed treatment.

In addition to such remarkable thick coating formation, components and hardness of a coating obtained by the electrical discharge coating treatment of the present invention are made uniform in the thickness direction of the coating, unlike a conventional electrical discharge coating that depends on carbide, ceramic, or the like. Thus, even when a secondary treatment such as grinding is performed, the surface form of the coating is not varied, and also a coating thickness large enough to withstand such treatment is secured.

With the features as described above, a valve seat conventionally press-fitted to an aluminum-alloy cylinder head may be replaced with a coating obtained by the electrical discharge coating treatment of the present invention, for example. This reduces restriction on the shapes of intake and exhaust ports of an engine and the shape of a combustion chamber. Thus, the present invention is advantageous for designing of the shapes producing a mixing flow inside a cylinder for swirling, tumbling or the like.

Moreover, since a coating obtained by the electrical discharge coating treatment of the present invention is metallurgically closely bonded to an aluminum material, a heat transfer loss is small at the interface of the coating and the aluminum material. This allows heat, received from the intake and exhaust valves, to be transferred efficiently to the aluminum-alloy cylinder head, and thereby inhibits localized overheating at valve surfaces of an inner surface of the combustion chamber. As a result, the compression limit of the engine can be improved. In addition, the fact that the components and hardness are uniform in the thickness direction brings about such an advantage that the sliding property, hardness, and the like of the surface of a valve seat do not change even when the surface is subjected to a secondary treatment such as grinding.

Thus, a coating obtained by the electrical discharge coating treatment of the present invention can be employed of course as an alternative coating for a valve seat of a cylinder head, and also as coating of surfaces of various metals and an alternative coating therefor which require wear resistance and heat resistance, such as an alternative coating for a cast iron sleeve of a cylinder block. Since masking by partial treatment is not necessary, there is an advantage in industrial use.

Hereinabove, some embodiments and examples of the present invention have been described. It is to be noted, however, that the present invention is not limited to these, and various modifications and changes are further possible on the basis of the technical concept of the present invention. 

1. An electrical discharge coating method comprising the steps of generating pulsed discharge between a green compact electrode and a treatment target surface in a working fluid; thereby, depositing a component of the green compact electrode onto the treatment target surface so as to form a coating, wherein the generating includes using the green compact electrode which is formed through compression molding of metal powder having an oxide layer on each particle surface thereof, as a main component thereof, the oxide layer being thicker than an oxide film normally obtained in air at normal temperature, whereby the metal component of the green compact electrode is deposited onto the treatment target surface so as to form the coating containing the metal component as a main component thereof.
 2. The electrical discharge coating method according to claim 1, wherein the metal powder is a powder of a metal selected from the group consisting of molybdenum, tungsten, chromium, molybdenum alloys, tungsten alloys, and chromium alloys.
 3. The electrical discharge coating method according to claim 1, wherein the metal powder is a mixed powder of two or more metals selected from the group consisting of molybdenum, tungsten, chromium, molybdenum alloys, tungsten alloys, and chromium alloys.
 4. The electrical discharge coating method according to any one of claims 1 to 3, wherein a percentage of the oxide layer of the metal powder is 5 to 11% by weight of the metal powder.
 5. The electrical discharge coating method according to any one of claims 1 to 4, wherein the treatment target surface is made of any material of aluminum, aluminum alloys, magnesium, magnesium alloys, titanium, and titanium alloys.
 6. A green compact electrode for an electrical discharge coating treatment, the treatment including generating pulsed discharge between the green compact electrode and a treatment target surface in a working fluid, and thereby a component of the green compact electrode is deposited onto the treatment target surface so as to form a coating, wherein the green compact electrode is formed through compression molding of metal powder, as a main component thereof, having an oxide layer on each particle surface thereof, the oxide layer being thicker than an oxide film normally obtained in air at normal temperature.
 7. The green compact electrode for an electrical discharge coating treatment according to claim 6, wherein the metal powder is a powder of a metal selected from the group consisting of molybdenum, tungsten, chromium, molybdenum alloys, tungsten alloys, and chromium alloys.
 8. The green compact electrode for an electrical discharge coating treatment according to claim 6, wherein the metal powder is a mixed powder of two or more metals selected from the group consisting of molybdenum, tungsten, chromium, molybdenum alloys, tungsten alloys, and chromium alloys.
 9. The green compact electrode for an electrical discharge coating treatment according to any one of claims 6 to 8, wherein a percentage of the oxide layer of the metal powder is 5 to 11% by weight of the metal powder.
 10. The green compact electrode for an electrical discharge coating treatment according to any one of claims 6 to 9, wherein the green compact electrode is formed through compression molding of the metal powder with a metal soap added thereto.
 11. The green compact electrode for an electrical discharge coating treatment according to any one of claims 6 to 10, wherein the green compact electrode is formed through compression molding of the metal powder with a metal powder added thereto, the added metal powder having a higher conductivity than that of the metal powder and not being subjected to an oxidation treatment.
 12. The green compact electrode for an electrical discharge coating treatment according to claim 11, wherein the higher-conductivity metal powder is any one of a copper powder and a silver powder.
 13. The green compact electrode according to any one of claims 10 to 12, wherein an amount of the metal soap added is 1 to 4% by weight of the metal powder.
 14. The green compact electrode according to any one of claims 10 to 12, wherein an amount of the higher-conductivity metal powder added is 2 to 4% by weight of the metal powder. 